High Vertical Stress Research Articles

The Coefficient of Earth Pressure at Rest K0 of Sands up to Very High Stresses

The mechanical behaviour of soils subjected to any stress path in which deviatoric stresses are present is heavily characterised by non-linearity, irreversibility and is strongly dependent on the initial state of stress. The latter, for the majority of geotechnical applications, is normally determined by the at-rest earth pressure coefficient K0, even though this state is valid, strictly speaking, for axisymmetric conditions and for zero-lateral deformations only. Many expressions are available in the literature for the determination of this coefficient for cohesive and granular materials both for normal consolidated and over-consolidated conditions. These relations are available for low to medium stress levels. Results of an extensive experimental investigation on two sands of different mineralogy up to very high stress (120 MPa) are reported in the paper. For reach very high vertical stresses, a special oedometer has been realised. In the loading phase (normal consolidated sands), the coefficient K0n depends on the stress level. It passes from values of about 0.8 to values of about 0.45 in the range of effective vertical stress σ′v = 0.5–4 MPa. Subsequently, K0n is about constant and varies between 0.45 to 0.55 up to very high vertical effective stresses (120 MPa). For the sands employed in the tests, Jaki’s relation did not lead to reliable results at relatively low pressures, while at high pressures, the same relationship seems to lead to reliable predictions if it refers to the constant volume angle of shear strength. For the over-consolidated sands, K0C strongly depends on the OCR, and for very high values of OCR, K0C could be greater than Rankine’s passive coefficient of earth pressure, Kp. This result is due to the very locked structure of the sands caused by the grain crushing, with intergranular contact of sutured and sigmoidal, concavo-convex and inter-penetrating type, that confer to the sand a sort of apparent cohesion and make it similar to weak sandstone.

Study on the influence of vertical stress difference coefficient on fracture characteristics of shale under high stress

AbstractTo study the effect of vertical stress difference coefficient on fracture characteristics of shale fracturing, high stress true triaxial hydraulic fracturing test was carried out. By analyzing the profile after fracturing, it was found that the area of hydraulic fracture increased with the increase of vertical stress difference coefficient, and the probability of shear fracture will increase when the stress difference coefficient was high. A high vertical stress differential coefficient exerts a strong control over the direction of crack propagation, while a low vertical stress difference coefficient is beneficial to improve the roughness of hydraulic fracture surface and promote the formation of complex fracture network. By analyzing the pump pressure curves of different tests, it was found that with the increase of vertical stress difference coefficient, the formation and expansion of hydraulic fractures were more difficult. The surface characteristics of hydraulic fractures were quantified based on three‐dimensional topography scanning technology, combined with fractal dimension and fracture area calculation method, the results showed that with the increase of vertical stress difference coefficient, the fractal dimension and fracture area decreased. Since shale is stratified, and the transformation of reservoir is mainly reflected in the enhancement of fracture complexity through tensile failure, Xsite discrete grid method was used to study the influence of fracture propagation behavior with different bedding strengths. The results showed that when the bedding tensile strength was high, hydraulic fractures were easy to pass through the bedding, and when the bedding tensile strength was low, hydraulic fractures were easy to be captured by natural fractures. In addition, tensile cracks were easy to form when the tensile strength of bedding was low, shear cracks were easy to form when the strength of bedding was high, and the fracture volume was larger when the strength of bedding was low. This study provides a theoretical basis for hydraulic fracturing in engineering.

Multiple-stage dynamic responses and failure behaviors of surrounding rocks subjected to development blasting: Exponential and triangular paths

During the development blasting of circular tunnels, the detonation of multiple blastholes arranged on concentric circles induces a complex dynamic response in the surrounding rocks. This process involves multiple blast loadings, static stress unloadings, and stress redistributions. In this study, the dynamic stresses of the surrounding rocks during development blasting, considering multiple blasting-unloading stages with exponential paths and triangular paths (linear simplified paths of exponential paths), are solved based on the dynamic theory and the Fourier transform method. Then, a corresponding discrete element model is established using particle flow code (PFC). The multiple-stage dynamic stress and fracture distribution under different in situ stress levels and lateral coefficients are investigated. Theoretical results indicate that the peak compressive stresses in the surrounding rocks induced by both triangular and exponential paths are equal, while the triangular path generates greater additional dynamic tensile stresses, particularly in the circumferential direction, compared to the exponential path. Numerical results show that the exponential path causes less dynamic circumferential tensile damage and forms fewer radial fractures than the triangular path in the first few blast stages; conversely, it exacerbates the damage and instability in the final blast unloading stage and forms more circumferential fractures. Furthermore, the in situ stress determines which of the two opposite effects is dominant. Therefore, when using overly simplified triangular paths to evaluate the stability of surrounding rocks, potential overestimation or underestimation caused by different failure mechanisms should be considered. Specifically, under high horizontal and vertical stresses, the static stress redistribution with layer-by-layer blasting suppresses dynamic circumferential tensile and radial compressive damage. The damage evolution of surrounding rocks in multi-stage blasting under different in situ stresses is summarized and classified according to the damage mechanism and characteristics, which can guide blasting and support design.

Floor Heave Control in Gob-Side Entry Retaining by Pillarless Coal Mining with Anti-Shear Pile Technology

The severe floor heave in gob-side entry retaining is the major restriction factor of the wide application of pillarless mining thin coal seams. Reinforcement and stress-relief floor heave control methods are the most promising. However, in practice, floor restoration is widely used. Therefore, floor heave control technology in gob-side entry retaining needs to be improved. This study proposes anti-shear pile technology to control floor heave in gob-side entry retaining. The research was mainly carried out by numerical simulation. It was found that the transformation of high vertical stresses in the entry floor underneath the filling wall and coal seam body into horizontal stresses starts the floor heave process. The vertical dilatancy of rocks under the roadway span and their subsequent unloading lead to the delamination of the floor strata and uplift of the entry contour. In this paper, the best pile installation scheme was found. It is a 2pile 5+2 scheme with the installation of two piles, each 2 m long. After that, it was shown that filling piles are more than 3.3 times cheaper than comparable analogs, and pile installation is less labor-intensive. The implementation of the proposed floor heave control method leads to a reduction in heaving by 2.47 times.

Geotechnical Characterisation of Flysch-Derived Colluvial Soils from a Pre-Alpine Slope Affected by Recurrent Landslides

Heterogeneous rock masses that include rhythmic alternations of marl, shale, marly limestone, sandstone, siltstone, and argillite, such as Flysch, are particularly prone to generating colluvial deposits on gentle slopes, which are often subject to failures triggered by heavy rainfall. Flysch-derived colluvial soils are made up of highly heterogeneous sediments ranging from clayey loam to rock fragments, and they have been studied more rarely than homogeneous soils. In this work, we present a geotechnical and hydraulic characterisation performed both in situ and in the laboratory on flysch-derived colluvial soils that were involved in a channelised landslide in the pre-alpine area of the Friuli Venezia Giulia region (NE Italy). The investigated soils were characterised by the average values of the grain size composition of about 25% gravel, 20% sand, 30% silt, and 25% clay. The loamy matrix presented low-to-medium values of the liquid and plastic limits, as well as of the plasticity index (LL = 40%, PL = 23%, and PI = 17%, respectively). The values of the peak friction angle for natural intact samples were 33° < ϕ’p < 38°, whereas the residual friction angle fell to 23–24° at great depths and high vertical stresses, for a prevailing silty–clayey matrix. Variable head permeability tests were performed both in situ and in the laboratory, showing that the values of the vertical and horizontal permeability were very close and in the range 1 × 10−4–1 × 10−6 m/s. The soil permeability measured in the field was generally higher than the hydraulic conductivity calculated on laboratory samples. The proposed geotechnical and hydrological characterisation of flysch-derived colluvial soils can be of fundamental importance before the use of more thorough analyses/models aimed at forecasting the possible occurrence of slope failures and evaluating the related landslide hazard. The reported geotechnical and hydraulic parameters of flysch-derived colluvial materials can represent a useful reference for rainfall infiltration modelling and slope stability analyses of colluvial covers that are subject to intense and/or prolonged precipitation. However, when facing engineering problems involving colluvial soils, particularly those coming from flysch rock masses, the intrinsic variability in their grain size composition, consistency, and plasticity characteristics is a key feature and attention should be paid to the proper assumption of the corresponding geotechnical and hydraulic parameters.

Mechanism of hydraulic fracture vertical propagation in deep shale formation based on elastic–plastic model

Deep shale reservoirs exhibit strong plasticity, and the simulation results from previous fracturing simulations based on elastic models may not adequately account for the effects of plastic. Therefore, the research objective is to develop a 3D finite element method (FEM) model of hydraulic fracture vertical propagation in multilayer reservoirs. The model simultaneously considering Drucker-Prager elastic–plastic rock model and bedding plane. To investigate the impact of various parameters on the vertical propagation pattern of fracture, it is crucial to consider factors such as friction angle, yield strength, interlayer strength, and vertical stress. This study quantitatively characterizes the mechanical parameters such as ground stress, matrix strength, and interface strength on hydraulic fracture morphology. Additionally, a new integrated control diagram is established to provide a comprehensive understanding of these effects. The results indicate that rocks with high friction angles and low yield strength are more resistant to cracking, and hydraulic fractures tend to have narrower fractures. Fracturing from high-strength and high-stress zones to lower zones promotes vertical fracture propagation. The high vertical stress plays a crucial role in preventing the fracturing fluid from entering the interface. Moreover, the strength of the formation interface greatly influences the vertical extension of hydraulic fractures. The bedding plane interface with a high cementation level can enhance the possibility of fracture vertical connection with multiple reservoirs. Conversely, breaking through low strength cemented interfaces in the formation can pose challenges for hydraulic fractures. The present simulation conditions determine that the fracture's ability to cross the layer depends on the ratio of interface strength to matrix strength, which should be between 0.25 and 0.5. These findings offer valuable theoretical guidance on the vertical propagation of fractures in deep multilayer fracturing.

Swelling behaviors of heterogeneous red-bed mudstone subjected to different vertical stresses

In this study, the axial swelling strain of red-bed mudstone under different vertical stresses are measured by swell-under-load method, and the microstructure of mudstone after hygroscopic swelling is studied by mercury intrusion porosimetry (MIP). The weakening coefficient and Weibull distribution function are introduced into the coupling model of mudstone moisture diffusion–swelling deformation–fracture based on finite–discrete element method (FDEM). The weakening effect of moisture on mudstone's mechanical parameters, as well as the heterogeneity of swelling deformation and stress distribution, is considered. The microcrack behavior and energy evolution of mudstone during hygroscopic swelling deformation under different vertical stresses are studied. The results show that the axial swelling strain of mudstone decreases with increase of the vertical stress. At low vertical stresses, moisture absorption in mudstone leads to formation of cracks caused by hydration-induced expansion. Under high vertical stresses, a muddy sealing zone forms on the mudstone surface, preventing further water infiltration. The simulation results of mudstone swelling deformation also demonstrate that it involves both swelling of the mudstone matrix and swelling caused by crack expansion. Notably, crack expansion plays a dominant role in mudstone swelling. With increasing vertical stress, the cracks in mudstone change from tensile cracks to shear cracks, resulting in a significant reduction in the total number of cracks. While the evolution of mudstone kinetic energy shows similarities under different vertical stresses, the evolution of strain energy varies significantly due to the presence of different types of cracks in the mudstone. The findings provide a theoretical basis for understanding the hygroscopic swelling deformation mechanism of red-bed mudstone at various depths.

Link between the Nankai underthrust turbidites and shallow slow earthquakes

Trench sediments such as pelagic clay or terrigenous turbidites have long been invoked to explain the seismogenic behavior of the megathrust fault (i.e., décollement). Recent numerous studies suggest that slow earthquakes may be associated with huge megathrust earthquake; however, controls on the slow earthquake occurrence remain poorly understood. We investigate seismic reflection data along the Nankai Trough subduction zone to understand the correlations between the spatial distribution of the broad turbidites and along-strike variations in shallow slow earthquakes and slip-deficit rates. This report presents a unique map of regional distribution of the three discrete Miocene turbidites that underthrust apparently along the décollement beneath the Nankai accretionary prism. A comparison of distributions of the Nankai underthrust turbidites, shallow slow earthquakes, and slip-deficit rates enables us to infer that the underthrust turbidites may cause primarily low pore-fluid overpressures and high effective vertical stresses across the décollement, leading to potentially inhibiting the slow earthquake occurrence. Our findings provide a new insight into potential role of the underthrust turbidites for shallow slow earthquakes at subduction zone.

Mechanistic formulation and validation of cutting seam by energy-focusing blast for pressure relief of deep roadway

Coal sources still comprise a significant proportion of the global energy supply, especially for developing countries. As mining depth increases, roadway instability, failure, and burst become increasing concerns over deep resource utilization and mining safety. Cutting seam by energy-focusing blast (CSEFB) technique is a novel approach to mitigate the problems. Driven by the demand for further understanding and application, this study discusses the mechanistic formulation of CSEFB through theoretical analysis, numerical simulation, and field experiments. Results show that CSEFB uses the broken-expansion rock and roof-cutting effect to relieve mining stresses. The CSEFB angle and length are two critical technical parameters. The angle is the prerequisite for validating pressure relief, and the length is the condition for determining the relief degree. The formulas are given for determining the range of angle and length. Within the range, the pressure-relief effect decreases modestly with increasing angle and increases significantly with increasing length. The standard length is proposed to use the support by bulking rock fully. An increase in the length can provide an additional effect by roof cutting itself, but this effect is limited as the length increases. This mechanism helps the formulation of CSEFB in the application. A bigger angle and the standard length are primary considerations. If requiring additional pressure-relief effect, a moderate increase in the length can add to pressure-relief effect by improving the stress field. Combined numerical simulation and field experiment with a typical high-stress deep mine, CSEFB exhibits good adaptability. Apart from relieving the high vertical stress, CSEFB avoids bad influences by the great horizontal stresses. The floor heave is exploited to facilitate the pressure-relief process. Research results have importance to the CSEFB application and promotion, thus contributing to the safety of deep mining.

Mechanical behaviors of deep pillar sandwiched between strong and weak layers

A variety of coal room and pillar mining methods have been efficiently practiced at depths of up to 500 m with least strata mechanics issues. However, for the first time, this method was trialled at depths of 850–900 m in CSM mine of Czech Republic. The rhomboid-shaped coal pillars with acute corners of 70°, surrounded with 5.2 m wide and 3.5–4.5 m high mine roadways, were used. Pillars were developed in a staggered manner with their size variation in the Panel II from 83 m × 25 m–24 m × 20 m (corner to corner) and Panel V from 35 m × 30 m–26 m × 16 m. Coal seam inclined at 12° was affected by the unusual slippery slickenside roof bands and sometimes in the floor levels with high vertical stress below strong and massive sandstone roof. In order to ensure safety, pillars in both the panels were continuously monitored using various geotechnical instruments measuring the induced stresses, side spalling and roof sagging. Both panels suffered high amounts of mining induced stress and pillar failure with side-spalling up to 5 m from all sides. Heavy fracturing of coal pillar sides was controlled by fully encapsulated steel bolts. Mining induced stress kept increasing with the progress of development of pillars and galleries. Instruments installed in the pillar failed to monitor actual induced stress due to fracturing of coal mass around it which created an apprehension of pillar failure up to its core due to high vertical mining induced stress. This risk was reduced by carrying out scientific studies including the three-dimensional numerical models calibrated with data from the instrumented pillar. An attempt has been made to study the behavior of coal pillars and their yielding characteristics at deeper cover based on field and simulation results.

Response in coal reservoirs and in-situ stress control during horizontal well coal cavern completion and stress release

Widely developed tectonically-deformed coal (TDC) blocks the recovery of coalbed methane (CBM). Horizontal well cavern completion and stress release (HWCCSR) is expected to extract TDC in-situ CBM efficiently. Systematic investigations on reservoir response during stress release are essential for evaluating stress-relief performances and guiding engineering practices. However, the dynamic-controllable cavern enlargement and the influence of high stress and intense tectonic stress were mainly ignored in previous work. Additionally, the mechanism of permeability variation remained unclear under complex stress surroundings. In this work, the geological-engineering numerical model of coal cavern completion was constructed, and the equivalent method was adopted to investigate the dynamic HWCCSR. The stress, plastic deformation, and permeability responses were analyzed, and the impact of in-situ stress on reservoir response was discussed. The mechanism and model of permeability changes under complex stress were revealed.The results showed that the equivalent method could imitate the dynamic-controllable enlarging of the cavern properly with easy operation and good convergence. The reservoir responded significantly during the HWCCSR. With increasing cavern diameter (Dc), the plastic deformation zone (PDZ) increased, permeability could be expanded to 1–105 times the initial value, and the effective permeability increase zone (EPIZ) was linear with Dc. Laboratory and field data confirmed the rationality of simulation results. High vertical principal stress and significant lateral stress coefficient were conducive to generating more complex PDZ and larger EPIZ and enhancing more considerable permeability in TDC reservoirs due to more energy and a strong damage-deformation trend. Four models of permeability variation and possible division of permeability distribution were proposed according to the integrated control of elastic-plastic deformation, stress difference, and volume stress. Stress difference was the direct reason for coal plastic deformed-failure. Plastic deformed-failure coupled with volume stress-relief could significantly increase permeability. Plastic deformed-failure and volume stress concentration might cause a permeability decrease. Coal elastic deformation with volume stress concentration might induce a similar outcome. Additionally, elastic deformation coupled with volume stress-relief might enhance permeability and expand the EPIZ under tectonic stress conditions. In conclusion, larger caverns had better stress-relief performance, expected to attain a high recovery of coalbed methane. The HWCCSR was suitable for recovering TDC in-situ CBM in areas with intense tectonic stress and large burial depths. This work confirmed the feasibility of efficient recovery of TDC in-situ CBM through HWCCSR. It provided basics for evaluating reservoir responses to stress-relief and selecting layers.

An energy-based quantitative coal burst proneness rating system for development roadways

Coal burst is one of the principal hazards in development panels of underground coal mines. This study develops a practical approach to quantitatively estimate coal burst proneness in development roadways. A rigorous numerical and statistical study is conducted to establish a novel burst proneness rating (BPR) system based on energy release characteristics. The rating system includes eight geological and geotechnical factors and covers a wide range of inputs. A dataset including 18 field cases (9 burst and 9 no-burst cases) was used to validate the proposed rating system and to determine a critical BPR value (i.e. the threshold value of burst cases). The probability hat{p}_{burst} of coal bursts was then obtained based on BPR logistic regression. The computed rating values using the BPR system can distinguish burst and no-burst cases that are in line with the scope of the eight input parameters in this study. The model results also showed that the mining depth was the most significant contributor of the energy release, along with roof elastic modulus and coal mass conditions. These results suggested that an environment that consists of relatively weak and brittle coal under high vertical stresses and surrounded by a relatively soft strata could promote unstable failures such as coal bursts.

Swelling and collapse behavior of expansive marl and its implications in foundation engineering

Expansive soils increase their volume upon wetting, but under high vertical stresses may exhibit collapse, with the latter aspect being rather overlooked in foundation engineering. Oedometer swelling/collapse tests on expansive Pliocene marls of various calcium carbonate content and plasticity index sampled from Nicosia, Cyprus, indicate that, unlike the free swelling strain, the swelling pressure is rather insensitive to the initial degree of saturation. Consequently, significant collapse strains upon wetting are not unlikely to occur in practical situations. The paper discusses the implications of this behavior in the case of shallow foundations and proposes methods of analysis. Under certain conditions, foundation design based on settlement calculations that use as input the fully saturated soil properties may be unconservative and supported structures may be exposed to severe risk of damage. For the design of mat foundations, stepwise distributions of the equivalent modulus of subgrade reaction across the mat as a function of the average bearing pressure are proposed based on the results of coupled finite element simulations. These distributions allow prediction of the peak bending moments produced at the end of the wet and dry seasons due to ground swelling and shrinkage, respectively, for the climatic conditions of Cyprus.

Numerical Study on Hydraulic Fracture Propagation in a Layered Continental Shale Reservoir

The distribution of beddings varies greatly in shale reservoirs. The influence of beddings on hydraulic fracture propagation has often been studied using simplified geological models, i.e., uniformly distributed bedding models. However, the propagation processes of hydraulic fractures in shale reservoirs with complicated distributed beddings remains unclear. In this research, an outcrop-data-based bedding model of a continental shale formation in the Ordos Basin, China, is built. A mathematical model for fracture propagation is built using the discrete element method and is then verified by a hydraulic fracturing experiment. Reservoir-scale simulations are employed to investigate the influence of geological factors and engineering factors on fracture geometry. The study finds that beddings have significant inhibitory effects on fracture height growth; hydraulic fractures have difficulty in breaking through zones with densely distributed beddings. If a hydraulic fracture encounters a bedding plane with a larger aperture, it is more likely to be captured and expand along the weak interface. High vertical stress difference and a high fluid injection rate can promote the vertical penetration of hydraulic fractures through beddings and activate the bedding system to yield a complex fracture network. Increments in fluid viscosity can increase the resistance of fracture propagation, thereby reducing fracture complexity.

Numerical Simulation of Hydraulic Fracturing and Penetration Law in Continental Shale Reservoirs

The vertical heterogeneity of continental shale reservoirs is strong, the difference between lithology and stress between layers is large, the weak interface between layers develops, and the hydraulic fracture penetration and expansion are difficult, resulting in poor fracturing transformation effect. In view of this, based on the finite element and cohesive element method, this paper established a fluid-solid coupling model for the hydraulic fracture propagation through the continental shale and studied the control mechanism and influence law of various geological and engineering parameters on the hydraulic fracture propagation through the continental shale reservoir using single factor and orthogonal test analysis methods. Interfacial cementation strength between high layers, high vertical stress difference, low interlaminar stress difference, low tensile strength difference, low elastic modulus difference, high pressure fracturing fluid viscosity, and high injection displacement are conducive to the penetration and expansion of hydraulic fractures. The primary and secondary order of influence degree of each factor is: interlaminar interface cementation strength > interlaminar stress difference/tensile strength difference > fracturing fluid viscosity/injection displacement > vertical stress difference > elastic modulus. In addition, engineering application research has also been carried out, and it is recommended that the injection displacement during early construction should not be less than 3 m3/min, and the fracturing viscosity should not be less than 45 mPa·s. The field application effect is good, which verifies the engineering application value of the model.

Full-Scale Pavement Testing of a High Polymer-Modified Asphalt Concrete Mixture

Abstract This paper presents the validation process for the structural layer coefficient determined mechanistically for high polymer (HP) asphalt mixtures (i.e., 0.54) through full-scale laboratory testing of asphalt pavements. Two instrumented full-scale pavement experiments were conducted in the PaveBox setup at the University of Nevada, Reno. A flexible pavement structure with a conventional polymer-modified asphalt layer was evaluated in Experiment No. 1. Meanwhile, a flexible pavement structure with a reduced-thickness HP asphalt concrete (AC) layer was evaluated in Experiment No. 2. Similar vertical surface deflections (except under the center of the applied surface load), higher vertical stresses at the middle of the base layer, and similar vertical stresses below the subgrade surface were observed with the reduced thickness of the HP AC layer. Moreover, better overall fatigue and rutting performance in the AC layer, slightly higher rut depths in the unbound layers, and similar total rut depths were observed by means of mechanistic analysis with the use of HP AC pavement. In general, the results and findings of this study supported the selection of 0.54.

On compression behavior and particle breakage of carbonate silty sands

Carbonate soils are high in compressibility and low in particle strength, and therefore need special treatment for many geotechnical applications. This paper presents comprehensive one-dimensional compression tests to investigate the compression behavior and particle breakage of carbonate soils with various initial particle size distributions (PSDs) and a wide range of initial void ratios at a high effective vertical stress of 32 MPa. A clear shape transition in terms of the void ratio against effective vertical stress was observed with varying silt fraction and void ratio. The deformation mechanism for the samples with less silt fraction and higher void ratio is mainly dominated by the particle-breakage-induced unlocking effect. Samples with more silt fraction and lower void ratio will experience less particle breakage and their deformation mechanism is mainly attributed to interparticle locking effect. A ‘transitional’ mode of behavior without a unique normal compression line was identified for the samples with silt fraction ranging from 1.6% to 33%. It was also found that carbonate soil samples with both ‘transitional’ and ‘non-transitional’ modes of compression behavior may both experience considerable amount of particle breakage and show different trends in the evolution of particle shape.

Tuning the stress state in Nb-thin films by lateral size confinement

For 5 nm thin Nb films adhered to a rigid substrate, hydrogen absorption leads to ultrahigh mechanical stress of about -8 GPa. This is related to a purely elastic behaviour. The high mechanical stress destabilizes phases and even suppresses conventional two-phase regions known for the Nb-H bulk system. These unique thin film properties can be preserved to thicker films, by lateral confinement. Nb-Fe films provide a laterally confined columnar domain structure of well-separated α-Nb(Fe) and μ-FeNb phases. While the α-Nb(Fe) absorbs hydrogen at low chemical potentials, the μ-FeNb phase does not, separating the two phases into a hydrogen active and a hydrogen passive one. The behaviour of 75 nm Nb-Fe films was studied by in situ mechanical stress measurements, in situ x-ray diffraction and transmission electron microscopy. With increasing Fe-content, Nb-Fe films show a strong increase in the yield stress upon hydrogen absorption. This allows for an extended elastic range and high maximum stress values of -4.6 GPa, for 75 nm Nb-Fe films. X-ray diffraction pattern collected upon hydrogen loading of Nb-Fe films show peak shifts and intermediate peak broadening. This indicates the presence of a two-phase region and preservation of the lattice coherency between the α-Nb(Fe)-H phase and the hydride phase. Peak shifts indicate a reduced vertical expansion of the hydrogen absorbing α-Nb(Fe) lattice. This is attributed to the presence of the µ-FeNb phase. FEM simulations on Nb-Fe-H films confirm reduced overall expansions and lateral stresses, when compared to pure Nb-H films. High vertical stresses arise upon H-loading due to the link between the two Nb-Fe phases. These stresses are tensile for µ-FeNb and compressive for the α-Nb(Fe) domains and also reach values of several GPa. Conservation of the exceptional thin film properties to thicker films by lateral confinement is suggested to be a powerful strategy for many different applications like sensor technologies or energy storage.

Water inrush and failure characteristics of coal seam floor over a confined aquifer

Failure behaviors of the floor rocks under coal seam mining in the conditions of hard magma rock roof and confined aquifer are studied. Based on the theory of rock stresses and elasticity mechanics, the combined effects of the abutment pressure induced by the hard roof and by the water pressure under the thin aquicludes of the floor rocks were considered, and a mechanical model was constructed along the strike of the working face. An analytical solution of stress distribution was derived in the floor rocks, the distributions of vertical, horizontal and shear stresses were calculated. In combination with the in-situ measurement, the results show that: 1) when the periodic pressure caused by the roof collapse occurs on the working face, and the maximum stress concentration in the floor appears at the elastic–plastic junction in the direction of the strike of the working face. With the increase of the depth of the floor, the horizontal stress coefficient tends to decrease, and the corresponding shear stress coefficient isoline shows a “symmetric spiral” distribution and propagates downward to the floor at a certain angle with the vertical direction. This causes the floor rocks to generate compression and shear or tension and shear failure. 2) when the immediate roof of coal seam is the magma rock, the abutment pressure shows a trend of a slow increase initially and then a rapid increase later. The peak value of abutment pressure appears at the location of 4–6 meters from the coal wall of the working face, and the concentration coefficient of the abutment pressure is between 1.4 and 1.8. 3) according to the measurement and calculation of the failure depths of the floor at different positions under the same coal seam, it is found that the maximum failure depth appears near the coal wall of the working face. The failure depth reduces by 11.6% after the floor goes through “the roof caving and re-compaction”, which causes the fractures in the floor to close and the thickness of the effective aquiclude increases. In the un-mined area of the working face, the failure depth is 55% of the maximum failure depth. 4) both the theoretical calculation and the numerical simulation show that the failure depth of the floor increases obviously under the combined action of high vertical stress and the water pressure. Under the condition that the thickness of the aquiclude is relatively thin, the water pressure of the floor and pressure intensity of the roof are the sensitive factors to affect the maximum failure depth of the floor. As the mining depth increases, the coal seam is getting closer to the Ordovician Limestone. This study provides an improved understand and theoretical support for the safe mining of coal seams under similar conditions.

Rib scaling and the stability of coal pillars

ABSTRACT The prediction of the depth of fracturing in the sides of underground excavations in coal seams is needed to determine the requirements for rib support and to assess pillar stability. The prediction requires consideration of how coal strength is determined and extrapolated to the mine scale and how stresses are redistributed around excavations in bedded coal masses. The impact of bedding in a stress analysis can be examined by adopting transverse isotropy and this results in much lower confining stresses in the horizontal plane and much higher vertical stresses close to the excavation boundary. The extent of rib scaling can be derived if the bedded coal is modelled as a brittle material. For coal pillars, the depth of scaling may be sufficient to have a material impact on their effective width and hence stability. The specification of rib support needs to consider how the failed coal may subsequently collapse.